The present invention relates to motion sensors for detecting movement in a room and activating lights or other apparatus accordingly.
A variety of sensors detect the presence of people in a room for the purpose of automatically turning on lights or other devices. Such sensors will also turn off the lights upon detecting that no one is in the room or area for a predetermined amount of time. One such system uses an ultrasonic transmitter and receiver, such as the one disclosed in U.S. Pat. No. 4,820,938. The ultrasonic signal received will show the presence of a person by the Doppler effect, i.e., the change in the frequency of the ultrasonic waves received, indicating movement of the person off whom the waves are bouncing. One disadvantage of this system is the false triggering that will occur when there are moving objects in the room, such as a vibrating air conditioning vent or other moving objects.
Another type of sensor uses an infrared sensor which detects the heat given off by a human in the room. Again, this technology is subject to false triggers due to the heat given off by other devices, such as computers, heating vents, etc.
Accordingly, it is desirable to have a motion sensor which overcomes the false triggering disadvantages of these prior art devices. In particular, such false triggering will keep the lights or other loads on when no one is in fact present, thereby resulting in added energy costs.